1. Field of the Invention
This invention generally relates to a system for controlling the connection condition of a plurality of terminals and in particular to a multi-terminal control system for controlling the connection condition of a plurality of data communication terminals connected to a communication path of the bus type in which signal and information channels are multiplexed.
2. Description of the Prior Art
Recently, there has been developed an Integrated Services Digital Network or simply ISDN, which is economical, high in quality and capable of providing a variety of services, by providing a digital communication network, such as telephone network, including a subscriber communication system, and its standardization process is being carried out by CCITT. The connection format between the network and a user's terminal apparatus in ISDN is of the bus type, and in accordance with the current CCITT recommendations, the user/network interface in ISDN has two kinds of interfaces, i.e., basic interface and primary group interface.
In the basic interface, up to eight terminal apparatuses can be connected to a single bus. Besides, the single bus is multiplexed with two B channels (64 K bps), each defining an information channel, and one D channel (16 K bps), which defines a signal channel for controlling a call connection with the network or the like. The two information channels may be used as a single information channel of 128 K bps, and the D channel may also be used as an information channel with some limitations. An example of the structure at the user side when use is made of the basic interface is illustrated in FIG. 4. That is, as shown in FIG. 4, a main interconnection bus MB1, MB2 is connected at the user side of each of network end terminal apparatus NT1, NT2 which provides the basic interface for connecting a terminal apparatus to the network in ISDN. To the main interconnection bus MB1 are connected two Group 4 facsimile machines (hereinafter, simply referred to as facsimile machines) GF11 and GF12 and a digital telephone unit DT1 through sockets ST11, ST12 and ST13, respectively. Similarly, to the main interconnection bus MB2 are connected two facsimile machines GF21 and GF22 and a digital telephone unit DT2 through sockets ST21, ST23 and ST22, respectively. It is to be noted that up to eight such ISDN terminal apparatuses may be connected to each of the main interconnection buses MB1 and MB2. It should also be noted that each of the main interconnection buses MB1 and MB2 is physically a four line transmission path with two circuits for transmission and reception, in which two information channels B1 and B2 (B channel) and one signal channel D are multiplexed.
For example, in the case where image information is to be transmitted from the facsimile machines GF11 to the facsimile machine GF21, in the first place, the facsimile machine GF11 must obtain the right of access to the signal channel D. And, using this signal channel D, the facsimile machine GF11 sends a request for connection to a destination to the network using the information channel B1 or B2. In this case, if the information channel B1 or B2 is not used by another terminal apparatus connected to the main interconnection bus MB1 and moreover the destination facsimile machine GF21 is not in communication, the network establishes a connection between the facsimile machines GF11 and GF21. And, thereafter, a predetermined Group 4 facsimile communication procedure is executed between the facsimile machines GF11 and GF21 so as to carry out transmission of image information. Upon completion of transmission of image information, the facsimile machine GF11 sends a request of call disconnection to the network, so that the transmission paths of facsimile machines GF11 and GF21 to the network are disconnected.
In the above-described basic interface, the maximum number of terminal apparatuses which may be connected to a single main interconnection bus is eight and there are provided only two information channels for each main interconnection bus, and thus, there is a case in which both of the two information channels are in use when one of the terminal apparatuses has requested a call connection to the network. In such a case, the terminal apparatus which has requested a call connection must wait until the data transmission of the terminal apparatus using these information channels is completed.